Add test stage to CI

.gitlab-ci.yml

 include: "https://git.autistici.org/ai3/build-container/raw/master/common.yml"
 
+stages:
+  - test
+  - build
+  - release
+
+test:
+  stage: test
+  image: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/golang:1.16
+  script:
+    - go test -v ./...